Cor Vink, an entomologist with the Biosecurity Group at AgResearch
and the Adjunct Curator of Spiders at the Entomology Research Museum
at Lincoln University, responded.

I’ll give you two answers to your question. First I’ll tell you how many
species of spider there are. Second, I’ll tell you how many actual individual
spiders there are.

There are about 2000 different species of spiders in New Zealand and most
of those species (93 per cent of them) are found nowhere else in the world.
For its size, New Zealand has a lot of spider species. The USA and Canada,
which are 74 times the size of New Zealand, have 3700 species. Only 1130
species of spider in New Zealand have been named and there are still over
800 species that are yet to be named. There are 42 000 named species of
spider in the world and probably another 130 000 species still to be named.

We can only estimate the actual numbers of individual spiders.
By looking at studies in which actual numbers of spiders have been counted,
there is an average of 180 spiders per square metre. Therefore there are
about 35 trillion (35 million million) individual spiders living in New Zealand.
Each year all those spiders would eat about 142 million tonnes of insects.
Compare this to the weight of the entire New Zealand population of 300 000 tonnes.
If it weren’t for spiders, everyone in New Zealand would have about 470 tonnes
of insects per person to cope with.